Surface treatment method for metal product and metal product

ABSTRACT

A surface treatment method capable of continuously forming a uniform nanocrystalline structure along the surface of a metal product regardless of whether the metal product is hard or soft. A substantially spherical spray powder that has a median diameter of 1-20 μm and a fall velocity in the air of 10 sec/m or more is sprayed onto a metal product at a spray pressure of 0.05-0.5 MPa. Thus, even when the metal product is made of a soft material, it is possible to form a uniform continuous nanocrystalline structure layer in which nanocrystals are micronized to an average crystal grain size of not more than 300 nm, preferably not more than 100 nm, without forming a laminar worked structure, impart a high compression residual stress of from about −180 MPa up to the order of −1200 MPa, and strengthen the surface of the metal product.

TECHNICAL FIELD

The present invention relates to a method for surface treatment of ametal article and to a metal article subjected to surface treatment bythe method. In particular, the present invention relates to a surfacetreatment method to strengthen a surface of a metal article by ejectingfine particles against the metal article under predetermined conditionsto make a crystal structure in the vicinity of the surface of the metalarticle to a nano-crystal structure, and to a metal article having asurface strengthened by such a method.

BACKGROUND OF THE INVENTION

The strength of a metal material being inversely proportional to thesquare root of crystal grain diameter is known as a Hall-Petchrelationship. Micronization of the crystal grain diameter to give suchan effect is also utilized in surface strengthening of metal articles.

In particular, a metal article having a crystal grain diameter in thevicinity of the surface micronized to nano crystal grain diameter notonly has dramatically increased surface hardness, but also has beenreported to achieve improved wear resistance and corrosion resistance.

As methods for nano-crystallization of metal articles enabling suchstrengthening of the surface, successful examples by ball milling,falling weight processing, particle colliding processing, and shotpeening has been reported. Especially, nano-crystallization by shotpeening is attracting particular attention due to being a low cost andeasy method.

Note that although there is still insufficient understanding of themechanism underlying the creation of a nano-crystal structure by shotpeening, examples of surface treatment are introduced in Patent Document1 and Non-Patent Document 1. The respective conditions therein aresurface treatment by ejecting shot made from high speed steel (SKH59)with an average particle diameter 45 μm at 0.5 MPa for 30 secondsagainst a soft material, in this case SS400 steel (HV 1.20 GPa (HV122));and surface treatment by shot peening under the same conditions againsta hard material, in this case SCr420 carburized and quenched steel(initial hardness HV 7.55 GPa (HV770)) in Patent Document 1 andNon-Patent Document 1. There are also descriptions therein of largedifferences between the nano-crystal structures formed in each example(see Patent Document 1 and Non-Patent Document 1).

Note that in the present specification conversion between HV (GPa) andHV (no units) is computed by “HV(no units)≈HV (GPa)×102” (see Table 1 ofJIS R 1610(2003)).

RELATED ARTS Patent Document

-   [Patent Document 1] Japanese Unexamined Patent Application    Publication No. 2007-297651

Non-Patent Document

-   [Non-Patent Document 1] “Formation of Nanocrystalline Structure by    Fine Particle Bombarding” by Shinichi Takagi and Masao Kumagai,    published in the Journal of the Japan Society for Precision    Engineering, Vol. 72, No. 9, 2006, pp 1079 to 1082.

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

As stated above, in Patent Document 1 and Non-Patent Document 1, whenattempting to generate nano-crystallization in the surface structure ofa metal article using shot peening, it has been reported that there is asignificant difference between nano-crystal structures (lamellarprocessing structures) created by treating a metal article configuredfrom a soft material, and nano-crystal structures (not accompanied withlamellar processing structures) created by treating a metal articleconfigured from a hard material.

From among these structures, nano-crystal structures created in thesurface of a metal article made from a hard material (SCr420 carburizedand quenched steel) are reported to be generated as nano-crystalstructures by a physical state and formed uniformly along the surface ina zone extending to a particular depth from the surface.

However, with a metal article made from a soft material (SS400 steel),significant indentations and protrusions are formed on the surface inthe initial stage of shot peening by colliding with ejection particles,as illustrated in FIG. 1. This is then followed by the protrusions, fromout of the indentations and protrusions formed by colliding withejection particles, being folded over toward the inside of the materialso as to be penetrated into the material. Repeatedly folding over of theprotrusions from out of further indentations and protrusions formed bysubsequent colliding then forms the lamellar processing structures,which have a layered structure resulting from multiple folded overlayers. The density of dislocations (strains) is increased significantlyin such lamellar processing structures, and this is interpreted asnano-crystallization when it exceeds a critical point.

The nano-crystal structures accompanying such lamellar processingstructures are not contiguously distributed along the surface of themetal article. Sometimes peripheral work-hardening regions are exposedat the surface, and sometimes the lamellar processing structures(nano-crystal structures) penetrate to positions deeper than thework-hardening regions. Moreover, when bonding during folding isinsufficient and inter-layer cracking has occurred to produce anon-uniform structure, this gives rise to a concern thatnano-crystallization induced by shot peening might actually result in adeterioration of the fatigue properties of the metal article. This isdue to the presence of surface portions where a nano-crystal structureis not formed and due to stress concentrating at portions where crackshave developed, etc.

Thus considering the point that when treatment is performed on a softmaterial in this manner, the nano-crystal structures that are createdalong with the lamellar processing structures do not enable surfacestrengthening to be performed, the treatment of Patent Document 1 islimited to treat metal articles made from hard steels having an initialhardness exceeding HV 7.0 GPa (HV714). There is no disclosure therein ofa method applicable to a soft material for forming a uniformnano-crystal structure continuously along the surface thereof.

The present invention accordingly solves the deficiencies of the relatedart described above. A first objective of the present invention is toprovide a method for surface treatment of a metal article in which thesurface treatment method is capable of forming a uniform nano-crystalstructure continuously along the surface of the metal article, withoutforming the lamellar processing structures described above, even whenthe metal article is made from a soft material.

A second objective of the present invention is to provide a surfacetreatment method of a metal article that is: capable of being appliedcommonly to metal articles spanning from those made of soft materials tothose made of hard materials, irrespective of the hardness of the basemetal of the metal article to be treated: and capable of forming auniform nano-crystal structure continuously along the surface of themetal article.

Note that in a cutting process performed using a cutting tool, thesurface of the workpiece is physically cut into and parted by thecutting-edge of the cutting tool, and a portion of the workpiece isscraped off. Performing cutting by continuously pressing-in thecutting-edge while removing the swarf (chip) generated by such scrapingleads to a high pressure being generated between the chip and the rakeface of the cutting tool. The accompanying large frictional resistanceand associated cutting heat physically and chemically changes the chipsuch that a portion of the chip accumulates to a leading portion of thecutting-edge. Accumulation formed by the chip accumulated to thecutting-edge of the cutting tool accordingly forms what is referred toas a “built-up edge”, which differs from the original cutting-edge.

Such built-up edge formation is not desirable due to it leading to adulling of the cutting-edge of the cutting tool, to a reduction inprocessing precision, and the like.

The accumulation of material to be processed typified by such a built-upedge is something that is not confined to cutting tools such as drills,end mills, hobs, broaches, milling cutters, and the like. Accumulationof material to be processed also occurs with cutting-edge portions ingeneral of machining tools that include a cutting-edge (edge) forcutting and parting, such as punching tools like punches.

However, applying the surface treatment method of the present inventionto cutting-edge portions of machining tools, as has been tried by theinventors of the present invention, has been demonstrated to improve themechanical properties of the cutting-edge portions, such as increase thehardness and improve the wear resistance thereof. In addition, thecapability of the surface treatment method to prevent material to beprocessed from accumulating to the cutting-edge portions, such as bysuppressing built-up edge generation, has also been confirmed.

Moreover, it is generally known that for sliding members, theslidability is improved by an effect in which oil is retained in dimplesformed by the ejection of, and collision by, particles. However, fordimples formed by treatment using a related treatment method, metal ispushed apart by collision with an abrasive, and the outer peripheries ofthe dimples are pushed up greatly into protruding shapes.

These protrusions at the outer peripheries of the dimples result in theinitial wear for a sliding member being raised. The protrusions at theouter periphery of the dimples are accordingly undesirable due tocausing cut metal to accumulate by initial wear, and due to causing adeterioration in the slidability such as abrasive wear and the like.

Such a phenomenon is generated for sliding members in general, such asbearings, shafts, gears, etc.

Applying the treatment of the present invention to sliding membersimparts hardness and residual stress to the sliding member. Thetreatment has, moreover, been confirmed to be a treatment method thatimproves the slidability, and makes the generation of projections lessliable to occur at the outer periphery of dimples which would raise theinitial wear of the sliding member.

Thus the present invention also has the objectives of: being utilized asa surface treatment method to prevent material to be processed fromaccumulating to cutting-edge portions of machining tools; and beingutilized as a surface treatment method to raise the hardness and impartresidual stress to sliding members, and to improve the slidability ofsliding members.

Means for Solving the Problems

In order to achieve the above objectives, a method for surface treatmentof a metal article according to the present invention is the methodcomprising:

ejecting substantially spherical ejection particles having a mediandiameter d50 of from 1 μm to 20 μm and a falling time through air of notless than 10 sec/m against a metal article at an ejection pressure offrom 0.05 MPa to 0.5 MPa;

forming a nano-crystal structure layer continuously along a surface ofthe metal article in a zone to a prescribed depth from the surface ofmetal article by uniform micronization to nano-crystals having anaverage crystal grain diameter of not greater than 300 nm; and impartingcompressive residual stress to the surface of the metal article.

“Median diameter d50” refers to the diameter at a cumulative mass 50percentile, namely, to a diameter that when employed as a particlediameter to divide a group of particles into two, results in the totalmass of particles in the group of particles of larger diameter being thesame as the total mass of particles in the group of particles of smallerdiameter. This is the same definition as “particle diameter at acumulative 50% point” in JIS R 6001 (1987).

In the above mentioned method for surface treatment of the metalarticle, preferably, the ejection velocity of the ejection particles isnot less than 80 m/sec.

Furthermore, the material of the metal article may be either aluminum oran aluminum alloy. In such case, the crystal grain diameter of thenano-crystal structure layer can be micronized to a crystal graindiameter not greater than 100 nm.

Furthermore, the metal article may be a machining tool, and a region tobe treated may be a cutting-edge (edge) of the machining tool and thevicinity of the cutting-edge, preferably, a range of at least 1 mm fromthe cutting edge, more preferably, a range of at least 5 mm from thecutting edge; and dimples having an equivalent diameter of from 1 μm to18 μm, preferably, 1 μm to 12 μm and a depth of from 0.02 μm to 1.0 μmor less than 1.0 μm may be formed on the region to be treated byejecting the ejection particles, such that a projected surface area ofthe dimples occupies not less than 30% of a surface area of the regionto be treated.

Moreover, the metal article may be a sliding member employed to slideagainst another member, such as a bearing, shaft, or gear, at least asliding portion of the sliding member is a region to be treated; anddimples having an equivalent diameter of from 1 μm to 18 μm, preferably,1 μm to 12 μm and a depth of from 0.02 μm to 1.0 μm or less than 1.0 μmmay be formed on the region to be treated by ejecting the ejectionparticles, such that a projected surface area of the dimples occupiesnot less than 30% of a surface area of the region to be treated. Notethat reference to “equivalent diameter” in the present invention refersto the diameter of a circle determined by converting the projectedsurface area for a single dimple formed on the region to be treated intoa circular surface area (“projected surface area” in the presentspecification means the surface area of the outline of the dimple).

Furthermore, a metal article according to the present invention is themetal article comprising: a base metal having a hardness not greaterthan HV714 (HV 7.0 GPa); a nano-crystal structure layer formedcontinuously along a surface of the metal article in a zone to aprescribed depth from the surface of metal article by uniformmicronization to nano-crystals having an average crystal grain diameterof not greater than 300 nm; and a compressive residual stress beingimparted to the surface of the metal article. Moreover, the metalarticle according to the present invention is configured from eitheraluminum or an aluminum alloy, and a crystal grain diameter of thenano-crystal structure layer is not greater than 100 nm.

Furthermore, the metal article may be a machining tool; the nano-crystalstructure layer may be formed on a surface of a region to be treatedincluding a cutting-edge and a vicinity of the cutting-edge; and dimpleshaving an equivalent diameter of from 1 μm to 18 μm and a depth of from0.02 μm to 1.0 μm or less than 1.0 μm may be formed such that aprojected surface area of the dimples occupies not less than 30% of asurface area of the region to be treated.

Moreover, the metal article may be a sliding member; the nano-crystalstructure layer may be formed on a surface of a sliding portion of thesliding member that makes sliding contact with another member: anddimples having an equivalent diameter of from 1 μm to 18 μm and a depthof from 0.02 μm to 1.0 μm or less than 1.0 μm may be formed such that aprojected surface area of the dimples occupies not less than 30% of asurface area of the region to be treated.

Effect of the Invention

By performing the surface treatment with the surface treatment method ofthe present invention as explained above, a uniform nano-crystalstructure layer can be formed continuously even on metal articles madefrom soft materials, in which hitherto it has not been possible to forma uniform nano-crystal structure layer continuously due to the formationof lamellar processing structures. Moreover, this surface treatment alsoimparts a high compressive residual stress equal to or higher than thatimparted when large ejection particles of comparatively large particlediameter are ejected at high ejection pressure.

Namely, ejection particles that have a small median diameter of from 1μm to 20 μm and have a falling time through air of not less than 10sec/m have a small mass. Although this means that stress is concentratedin the vicinity of the surface of the metal article and does notpropagate deeply, the surface deformation of the metal article on beingcollided can also be made small. Such ejection particles are easilycarried on an airflow, and can therefore be propelled at a velocityclose to the airflow velocity. This enables such ejection particles tobe ejected at similar velocities to the velocity of airflow flowinginside an ejection nozzle, at velocities of 80 m/sec or greater, forexample.

As a result, the colliding energy required to obtain nano-crystalstructures can be achieved even when ejecting with a comparatively lowejection pressure of about 0.05 MPa. The surface hardness increasingeffect on a metal article is substantially saturated when the ejectionpressure is about 0.1 MPa, and there is substantially no furtherincrease in hardness observed from ejecting at ejection pressures of 0.1MPa and greater. Nano-crystal structures can be obtained irrespective ofthe base metal hardness of the metal article even with comparativelyweak ejection pressures not exceeding 0.5 MPa. Compressive residualstress can also be imparted therewith that is of the same level to whenejection particles of 50 μm or greater are ejected at high pressure asdescribed in the related art.

Moreover, about 60% of the hardness and compressive residual stress thatresulted from an ejection pressure of 0.1 MPa could also be confirmed atan ejection pressure of 0.05 MPa.

As a result, the lamellar processing structures such as those explainedwith reference to FIG. 1 are not formed even for metal articles madefrom soft materials such as aluminum alloys. This thereby enables anano-crystal structure layer to be formed uniformly and continuously.This is thought to enable a nano-crystal structure layer to be formeduniformly and continuously using a lower ejection pressure than theejection pressure indicated in the related art documents, even for ametal article made from a hard material.

Moreover, due to being able to perform surface treatment on metalarticles under the same treatment conditions irrespective of thehardness of the base metal of the metal article, as described above,this enables nano-crystallization to be performed without ascertainingin advance the hardness or the like of the metal article to be treated.This enables surface treatment to be performed continuously, such as ona conveyor line conveying plural types of metal article made fromdifferent materials etc.

Moreover, the surface treatment method of the present invention enablesa uniform nano-crystal structure layer to be formed continuously along asurface without forming the lamellar processing structures describedabove, even for metal articles made from aluminum or aluminum alloys,which have particularly low hardness from among metal materials. Due tobeing able to achieve a finer crystal grain diameter of 100 nm or lessfor the nano-crystal structure layer formed when treating aluminum or analuminum alloy, a higher degree of surface strengthening effect can beobtained.

Moreover, consider an example in which the region to be treated is acutting-edge (edge) of a machining tool such as a cutting tool and inthe vicinity of the cutting-edge, and the equivalent diameter of dimplesformed by the ejection of ejection particles onto the region to betreated is from 1 μm to 18 μm, and preferably from 1 μm to 12 μm, thedepth of such dimples is from 0.02 μm to 1.0 μm or less than 1.0 μm, andthe projected surface area of such dimples is not less than 30% of thesurface area of the region to be treated. In such an example, not onlyis the generation of built-up edge or the like at the cutting-edgeprevented, and the cutting-edge of the treated machining toolstrengthened, but the material to be processed can also be preventedfrom accumulating to the cutting-edge.

Thus employing the method of the present invention to treat a slidingmember enables the height of protrusions formed at the outer peripheriesof the dimples to be suppressed, and enables the slidability to beimproved by preventing abrasive wear and accumulation of abraded powder,etc. due to reducing initial wear.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram illustrating a mechanism by whichlamellar processing structures are formed in a soft material.

FIG. 2 are explanatory diagrams illustrating an example of applicationto a cutting-edge of a machining tool: (A) illustrates a state beforetreatment, and (B) illustrates a state after treatment.

FIG. 3 is an explanatory diagram of a portion (pressure receivingsurface) where compressional force acts when collided by an ejectionparticle.

FIG. 4 is a Von Mises stress analysis image using FEM (5 μm ejectionparticles).

FIG. 5 is a Von Mises stress analysis image using FEM (10 μm ejectionparticles).

FIG. 6 is a Von Mises stress analysis image using FEM (20 μm ejectionparticles).

FIG. 7 is a Von Mises stress analysis image using FEM (50 μm ejectionparticles).

FIG. 8 is a Von Mises stress analysis image using FEM (100 μm ejectionparticles).

FIG. 9 is a graph illustrating a relationship between particle diameterof ejection particles and stress.

FIG. 10 is a graph illustrating a relationship between particle diameterof ejection particles and depth of maximum stress generation.

FIG. 11 is a graph illustrating relationships between ejection pressureand dynamic hardness.

FIG. 12 are SIM images of pre-hardened steel (“NAK 80”, manufactured byDaido Steel Co., Ltd): (A) illustrates a state before treatment, and (B)illustrates a state after the treatment of the present invention.

FIG. 13 are SIM images of an alloy tool steel (SKD11): (A) illustrates astate before treatment, and (B) illustrates a state after treatment ofthe present invention.

FIG. 14 are SIM images of an aluminum alloy (A7075): (A) illustrates astate before treatment, and (B) illustrates a state after treatment ofthe present invention.

FIG. 15 is a grain diameter distribution diagram for pre-hardened steel(“NAK 80”, manufactured by Daido Steel Co., Ltd) treated by the methodof the present invention.

FIG. 16 is a grain diameter distribution diagram for alloy tool steel(SKD11) treated by the method of the present invention.

FIG. 17 is a graph of measurement results of residual stress inpre-hardened steel (“NAK 80”, manufactured by Daido Steel Co., Ltd).

FIG. 18 is graph of measurement results of residual stress in alloy toolsteel (SKD11).

FIG. 19 is a graph of measurement results of residual stress in aluminumalloy (A7075).

FIG. 20 is a graph of measured changes in friction with respect toelapsed time.

DESCRIPTION OF EMBODIMENTS

Next, explanation follows regarding an embodiment of the presentinvention, with reference to the appended drawings.

Object to be Treated

A metal article subjected to treatment by the surface treatment methodof the present invention may be any article made from metal, and, aswell as application to ferrous metals, application may also be made tometal articles made from non-ferrous metals and alloys thereof.

Moreover, the metal article to be treated is not limited to a metalarticle configured from a hard base metal, and application may be madeto a range of metals from comparatively soft metals of about HV20 toHV400 such as aluminum and alloys thereof, pre-hardened steels (“NAK80”, manufactured by Daido Steel Co., Ltd: HV400) and the like, up tohigh hardness steels, such as SKD11 (HV697).

In particular, the method of the present invention is able to treatmetal articles made from soft materials, in which hitherto it has beenimpossible to form a nano-crystal structured layer uniformly andcontinuously due to the formation of lamellar processing structures asexplained with reference to FIG. 1. From among such soft materials, ithas been confirmed that the method can achieve a nano-crystal structurelayer formed with an extremely fine crystal grain diameter, this being acrystal grain diameter of 100 nm or less, when metal articles made fromaluminum and aluminum alloys, which have a particularly low hardness,are treated. A large surface strengthening effect can be obtained as aresult.

Note that there are no particular limitations to the usage applicationof the treated metal article, and application may be made to metalarticles employed in various applications requiring surfacestrengthening. However, a preferable application of the surfacetreatment method of the present invention is application to acutting-edge of a machining tool such as cutting tool, or to thevicinity of the cutting-edge. This is due to not only being able tostrengthen the cutting-edge portion, but also being able to prevent thematerial to be processed from accumulating to the cutting-edge.

When performing treatment on a cutting-edge of a machining tool in thismanner, ejection particles described later are ejected to the region tobe treated where the ejection particles are ejected and caused to becollided thereto, i.e., a portion of the cutting-edge (edge) asillustrated in FIG. 2 where shearing starts when cutting or shearing,and a range of at least 1 mm from the cutting-edge, and preferably arange of at least 5 mm from the cutting edge (the range from thecutting-edge indicated by the double-dashed broken lines in thedrawings). Dimples are also formed in this region accompanying theformation of a nano-crystal structure layer on the surface of thisportion, as illustrated in FIG. 2(B).

In the present embodiment, inclined faces on either side of thecutting-edge may be employed as the region to be treated. However, theregion to be treated may be solely provided on the inclined face thatbears the greatest frictional resistance during cutting, or solelyprovided on the inclined face on the side that cut material might beaccumulated thereto.

Furthermore, when performing the surface treatment method of the presentinvention with the objective of surface strengthening and improving theslidability of a sliding member employed to slide against anothermember, such as a bearing, shaft, or gear, the region to be treatedreferred to above is at least a portion of the sliding member thatslides against the other member.

Note that the surface of the metal article to be treated may be in aburred state, or may be in a state in which processing marks such astool marks remain formed thereon. However, preferably pre-polishing isperformed in advance to polish to surface roughness having an arithmeticmean roughness (Ra) of 3.2 μm or less.

There are no particular limitations to the method by which suchpre-polishing is performed, and polishing may be performed by manuallapping or buffing. However, such pre-processing is preferably performedby blasting using an elastic abrasive.

Such an elastic abrasive is an abrasive having abrasive particlesdispersed in an elastic body such as a rubber or an elastomer, or is anabrasive having abrasive particles supported on the surface of anelastic body. Such an elastic abrasive can be caused to slide across thesurface of a metal article by being ejected at an inclination thereto,or the like. The surface of the metal article can thereby becomparatively simply polished to a mirror finish, or polished to a stateclose to a mirror finish.

The abrasive particles dispersed in, or supported by, the elastic bodyof the elastic abrasive may be appropriately selected according to thesurface state of the metal article etc. An example of abrasive particlesthat may be employed therefor are silicon carbide and diamond abrasiveparticles of from 1000 grit to 10000 grit.

Surface Treatment

Substantially spherical ejection particles are ejected against theregions described above of the surface of the metal article wheresurface strengthening is to be performed, and are caused to collidethese regions.

Examples of the ejection particles, ejection apparatus, and ejectionconditions employed when performing the above surface treatment aregiven below.

(1) Ejection Particles

For the substantially spherical ejection particles employed in thesurface treatment method of the present invention, “substantiallyspherical” means that they do not need to be strictly “spherical”, andordinary “shot” may be employed therefor. Particles of any non-angularshape, such as an elliptical shape and a barrel shape, are included in“substantially spherical ejection particles” employed in the presentinvention.

Materials that may be employed for the ejection particles include bothmetal-based and ceramic-based materials. Examples of materials formetal-based ejection particles include steel alloys, cast iron,high-speed tool steels (HSS) (SKH), tungsten (W), stainless steels(SUS), boron (B), chromium boron steels (FeCrB), and the like. Examplesof materials for ceramic-based ejection particles include alumina(Al₂O₃), zirconia (ZrO₂), zircon (ZrSiO₄), hard glass, glass, siliconcarbide (SiC), and the like.

Regarding the particle diameter of the ejection particles employed,particles having a median diameter (d50) in a range of from 1 μm to 20μm may be employed. Iron-based ejection particles that may be employedhave a median diameter (d50) in a range of from 1 μm to 20 μm, andpreferably in a range of from 5 μm to 20 μm. Ceramic-based ejectionparticles that may be employed have a median diameter (d50) in a rangeof from 1 μm to 20 μm, and preferably in a range of from 4 μm to 16 μm.

For fine powder ejection particles having a median diameter from 1 μm to20 μm, the ejection particles can be imparted with the property ofhaving a long falling time through air (caused to float in air) byselecting a material density of the ejection particles. Ejectionparticles having such properties readily ride on an airflow, and can bepropelled with a velocity similar to that the airflow velocity.

In the surface treatment method of the present invention, the ejectionparticles employed have a falling time in still air conditions of 10sec/m or greater. This enables the ejection particles to be ejected atsubstantially the same velocity as the velocity of an airflow beingejected from an ejection nozzle of a blasting apparatus.

With regard to the falling speed, for the same particle diameter, thefalling time is longer, the lower the density of the materialconfiguring the ejection particles. For iron-based ejection particleshaving a relative density (specific gravity) of 7.85, the falling timeis 10.6 sec for a particle diameter of 20 μm, and 41.7 sec for aparticle diameter of 10 μm. For ceramic-based ejection particles havinga relative density of 3.2, the falling time is 26.3 sec for a particlediameter of 20 μm, and 100 sec for a particle diameter of 10 μm.

Note that the ejection particles employed are preferably ejectionparticles of a material having a hardness equivalent to or greater thanthat of the base metal of the metal article to be treated. Whenceramic-based ejection particles are employed, the ejection particleshave a higher hardness than substantially all metal articles. Thedensity of ceramic-based ejection particles is also low, and the fallingtime as described above is long. This means that ceramic-based ejectionparticles are preferably employed due to being able to obtain a highejection velocity.

(2) Ejection Apparatus

A known blasting apparatus for ejecting abrasive together with acompressed gas may be employed as the ejection apparatus to eject theejection particles described above toward the surface of region to betreated.

Such blasting apparatuses are commercially available, such as a suctiontype blasting apparatus that ejects abrasive using a negative pressuregenerated by ejecting compressed gas, a gravity type blasting apparatusthat causes abrasive falling from an abrasive tank to be carried bycompressed gas and ejected, a direct pressure type blasting apparatus inwhich compressed gas is introduced into a tank filled with abrasive andthe abrasive is ejected by merging the abrasive flow from the abrasivetank with a compressed gas flow from a separately provided compressedgas supply source, and a blower type blasting apparatus that carries andejects the compressed gas flow from such a direct pressure type blastingapparatus with a gas flow generated by a blower unit. Any one of theabove may be employed to eject the ejection particles described above.

(3) Treatment Conditions

Substantially spherical ejection particles configured from one of thematerials described above or the like, and having a median diameter d50of from 1 μm to 20 μm and a falling time through air of not less than 10sec/m are ejected against the metal article as described above at anejection pressure of from 0.05 MPa to 0.5 MPa.

Confirmation of Optimum Conditions

(1) Diameter of Ejection Particle

(1-1) Concept

As described above, the lamellar processing structures as explained withreference to FIG. 1 need to be suppressed from being generated in orderto form a uniform nano-crystal structure layer continuously along asurface of a metal article made from a soft material. In order tosuppress the generation of such lamellar processing structures,deformation of the metal article surface needs to be suppressed fromoccurring when collided by the ejection particles.

On the other hand, it is considered that strain exceeding a criticalvalue needs to be imparted in the vicinity of the surface of the metalarticle in order to generate nano-crystal structures, and that a largecolliding force needs to be imparted to the surface of the metal articleby collision of the ejection particles in order to impart strainexceeding the critical value.

However, the larger the colliding force imparted to the surface of themetal article, the larger the amount of deformation at the surface ofthe metal article, and the more readily the lamellar processingstructures explained with reference to FIG. 1 are generated. This makesit difficult for a metal article made from a soft material to generate auniform nano-crystal structure layer continuously across the surfacewithout being accompanied by lamellar processing structures.

The inventors of the present invention have accordingly investigatedtreatment conditions that enable these conflicting demands to besatisfied, i.e. the need to reduce the colliding force received by themetal article surface when collided by the ejection particles tosuppress deformation of the surface of the metal article, with the needto also impart strain exceeding the critical value required to generatethe nano-crystal structures.

(1-2) Deformation Amount by Collision

The deformation amount generated at the metal article surface whencollided by the ejection particles has been investigated.

Particles having a median diameter d50 of from 20 μm to 40 μm werecaused to collide a surface, and the volume of protrusions on thesurface was measured using a profile analyzing laser microscope. Acomparison was then made between the protrusion volume and the ease ofgeneration of the lamellar processing structures formed by folding. Thiswas done because it was thought that the larger the protrusion volume,the larger the amount of folding that would be generated when collidedby the particles.

A profile analyzing laser microscope (“VK-X250”, manufactured by KeyenceCorporation) was employed as the measuring method, and measurements weretaken of the surface at a measurement magnification of 1000×.

The measured data was analyzed using a Multi-File Analysis Application(manufactured by Keyence Corporation).

The Multi-File Analysis Application is software that uses data measuredby a laser microscope to perform various measurements, such as surfaceroughness, flatness measurements, profile measurements, volume/areameasurements, etc.

In measuring, first the “image processing” function was used to set thereference plane (however, in cases in which the surface shape is acurved plane, the reference plane is set after the curved plane has beencorrected to a flat plane by using plane shape correction). Then, themeasurement mode was set to protrusion in the “volume/area measurement”function of the application, protrusions were measured with respect tothe set “reference plane”, and the average value of the “volume” in theprotrusion measurement results was set as a dimple protrusion volume.

Note that the reference plane described above was computed from heightdata using a least squares method.

These results are given in the following table (Table 1). The particlediameter of 20 μm of the scope of the present invention resulted in aprotrusion volume that was about 70% less than that resulting from a 40μm diameter in related art. It was thought that this extremely smalldeformation amount was a reason for the uniform nano-crystal structureformation.

TABLE 1 Ejection Particle Diameter and Protrusion Volume ExampleComparative Example Ejection particle 20 40 diameter (D50) μm Protrusionvolume μm³ 932 2738

(1-3) Investigation of Colliding Force F

When the above treatment conditions were investigated, the relationshipbetween the colliding force F and the ejection particle diameter wasre-investigated based on computation equations to compute the collidingforce F imparted to the surface of the metal article by colliding withan ejection particle (1 particle).

When the mass of an ejection particle (1 particle) is m (kg), thevelocity of the ejection particle before impact is v1 (m/sec), thevelocity of the ejection particle after impact is v2 (m/sec), and thecoefficient of restitution ε on impact is assumed to be 1.0, then amomentum M1 of the ejection particle before impact, and the momentum M2after impact, are given by the following equations:

M1=m·v1 (kgm/s)  Equation 1

M2=m·v2 (kgm/s)  Equation 2

Thus a change in momentum ΔM of the ejection particle between before andafter impact is:

ΔM=M1−M2=m·v1−(−m·v1)=2m·v1 (kgm/s)  Equation 3

The change in momentum ΔM here is equivalent to the impulse FΔt (whereinΔt is the duration of impulse).

FΔt=ΔM  Equation 4

Thus, the colliding force F imparted to the surface of the metal articlewhen collided by the ejection particle (1 particle) is:

F=ΔM/Δt=2mv1/Δt(N)  Equation 5

According to the colliding force F of Equation 5, the colliding force Fchanges in proportional to a mass m of the ejection particle, and so thecolliding force F gets larger as the ejection particle diameterincreases.

(1-4) Ejection Particle Diameter and Pressure Receiving Surface

As the particle diameter of the ejection particle increases, thecolliding force F also increases, as described above. Thus if theparticle diameter of the ejection particles employed is large and thecolliding force F is large, the surface area of the portion of the metalarticle surface undergoing deformation (the portion indicated by thereference sign S in FIG. 3) also increases when the surface of the metalarticle is collided with the ejection particles.

The way in which the surface area (pressure receiving surface S) actedon by compressional force on the metal article surface changes wasinvestigated by changing the particle diameter of such ejectionparticles.

Taking the surface of the metal article where interaction with theejection particles occurs (a circular shape horizontal plane) as apressure receiving surface S, then relationships expressed by Equation 6and Equation 7 below are satisfied between a radius a of the pressurereceiving surface S, a radius r of the ejection particles, and a depth Xof the depressions:

a ²+(r−X)² =r ²  Equation 6

a ² =r ²−(r−X)² =r ²−(r ²−2rX+X ²)=2rX−X ²  Equation 7

Wherein, taking a as a ratio of a depth X of the depressions to adiameter d of the ejection particles, then:

X=2rα  Equation 8

Thus substituting Equation 8 for X in Equation 7 gives:

a ²=2r(2rα)−(2rα)²  Equation 9

Since 2r=d:

a ² =d ² α−d ²α² =d ²α(1−α)  Equation 10

Thus, a surface area S (m²) of the pressure receiving surface is givenby Equation 10.

S=πα ² =πd ²α(1−α)  Equation 11

Equation 11 shows that the surface area of the pressure receivingsurface S increases in proportional to the square of the diameter of theejection particles.

With regard to the lamellar processing structures explained above withreference to FIG. 1, indentations and protrusions are formed duringcolliding, and then the protrusions from out of these indentations andprotrusions are folded over to form the lamellar processing structures.These protrusions are formed by base metal at the depression portionsexplained with reference to FIG. 3 (the shaded portion in FIG. 3) beingpushed out when collided by an ejection particle.

Thus, as the surface area of the pressure receiving surface S describedabove increases, the protrusions formed become larger, and this ispostulated to facilitate the formation of the lamellar processingstructures.

(1-5) Ejection Particle Diameter and Ejection Velocity

From the above equation of colliding force F (Equation 5), the collidingforce F does not only increase with an increase in mass m of theejection particles, but also increases with an increase in the ejectionvelocity v1.

The ejection velocity was computed with reference to ejection velocitycomputation equations in a paper regarding how the ejection velocitychanges with respect to changes in particle diameters of ejectionparticles: “Measurement and Analysis of Shot Velocity in Pneumatic ShotPeening” by Ogawa, Asano, et al (Transactions of the Japan Society ofMechanical Engineers, Edition C. Volume 60. No. 571, 1994-3).

(1-6) Predicting Optimum Particle Diameters for Ejection Particles

The above computation equations and the like were employed, and thechange in colliding conditions to changes in particle diameter for steelejection particles (relative density of 7.85) as an example, aresummarized in Table 2 and Table 3, below.

TABLE 2 Change in Colliding Conditions with Changes to Ejection ParticleDiameter (Ejection Pressure: 0:5 MPa) Colliding Pressure force F/Ejection Receiving Pressure Number of Particle Velocity Impulse SurfaceReceiving Ejection Colliding diameter Mass m V1 Duration Δt CollidingArea S Surface Particles Energy (μm) (μg) (m/sec) (μs) force F (kgf)(mm²) Area S (per kg) (J) 5 0.0005 245 10 2.57 × 10⁻⁶ 2.35 × 10⁻⁷ 10.91.95 × 10¹⁵ 3.00 × 10⁷ 10 0.0041 245 20 1.03 × 10⁻⁵ 9.40 × 10⁻⁷ 10.92.43 × 10¹⁴ 3.00 × 10⁷ 20 0.0329 198 40 3.17 × 10⁻⁶ 3.76 × 10⁻⁶ 8.4 3.04× 10¹³ 1.79 × 10⁷ 50 0.5138 150 100 1.57 × 10⁻⁴ 2.35 × 10⁻⁵ 6.7 1.95 ×10¹² 1.13 × 10⁷ 100 4.1103 130 200 5.45 × 10⁻⁴ 9.40 × 10⁻⁵ 5.8 2.43 ×10¹¹ 8.45 × 10⁶

TABLE 3 Change in Collision Conditions with Ejection Particle Diameter(Ejection Pressure: 0.05 MPa) Pressure Colliding Number EjectionReceiving force F/ of Particle Velocity Impulse Colliding SurfacePressure Ejection diameter Mass m V1 Duration Δt force F Area SReceiving Particles Colliding (μm) (μg) (m/sec) (μs) (kgf) (mm²) SurfaceArea S (per kg) Energy (J) 5 0.0005 112 10 1.17 × 10⁻⁶ 2.35 × 10⁻⁷ 5.01.95 × 10¹⁵ 6.27 × 10⁶ 10 0.0041 112 20 4.47 × 10⁻⁶ 9.40 × 10⁻⁷ 5.0 2.43× 10¹⁴ 6.27 × 10⁶ 20 0.0329 86 40 1.44 × 10⁻⁵ 3.76 × 10⁻⁶ 3.8 3.04 ×10¹³ 3.70 × 10⁶ 50 0.5138 61 100 6.40 × 10⁻⁵ 2.35 × 10⁻⁵ 2.7 1.95 × 10¹²1.86 × 10⁶ 100 4.1103 47 200 1.97 × 10⁻⁴ 9.40 × 10⁻⁵ 2.1 2.43 × 10¹¹1.10 × 10⁶

As is apparent from Table 2 and Table 3, the colliding force F increasesthe larger the particle diameter of the ejection particles, however,accompanying such increases, the pressure receiving surface area S alsoincreases. As a result, large protrusions are formed on the surface ofthe metal article being collided with the ejection particles. This isthought to facilitate generation of the lamellar processing structures,which are thought to be generated by folding such protrusions.

Moreover, the larger the particle diameter of the ejection particles,the larger the value of the colliding force F. However, the surface areaof the pressure receiving surface S increases in proportional to thesquare of the diameter d of the ejection particles, as stated above.This means that when the colliding force F per unit surface area of thepressure receiving surface S (colliding force F/pressure receivingsurface area S) is considered, then the force imparted per unit surfacearea actually decreases.

With regard to colliding energy, in cases in which each of the particlesin Table 1 are ejected at 0.5 MPa, if the colliding energy when theparticle diameter d50=50 μm is taken as 1, then the particles withd50=10 μm and 20 μm can be projected against a surface with highenergies of about two to three times this energy.

Thus when ejection particles of large diameter are employed, not only isthe surface of the metal article is more readily deformed, facilitatingthe generation of lamellar processing structures as explained withreference to FIG. 1, but this is also conjectured to make it difficultto obtain a strain exceeding the critical value required to obtainnano-crystal structures.

A simulation of Von Mises stress was accordingly performed by analysisusing a finite element method (FEM) (referred to below as FEM analysis)based on the computed values given in Table 2 and Table 3. These resultsare illustrated in FIG. 4 to FIG. 8.

Moreover, the results obtained from this simulation are illustrated as agraph in FIG. 9 of a relationship between change in stress and ejectionparticle diameter, and as a graph in FIG. 10 of a relationship betweendepth at which the maximum stress is generated and ejection particlediameter.

FEM analysis is a numerical analysis method for use in cases difficultto solve by analytical methods such as complex geometric models. In FEManalysis, an area is divided into finite elements, simple formulae areestablished at the element level, and a solution for the whole system isobtained by using interpolation functions between elements to make anapproximation thereof. “Femap with NX Nastran” (sold by NST Co., Ltd.)was employed as analysis software.

Moreover, “Von Mises stress” is equivalent stress based on shear strainenergy theory. Von Mises stress is expressed as a scalar value withoutdirectionality, and in a stress field where complex loading acts in inplural directions, the Von Mises stress is a value for uniaxial tensionor compressive stress.

The Von Mises stress is referenced as an indicator to determine whetheror not a given material will yield. This means that there is no need tolook at stress in other directions when comparing against yield stress,and yield determination is made using a single Von Mises stress. Thiswas utilized to simulate stress arising from colliding with the ejectionparticles.

It is apparent from looking at the simulation results between particlediameter of ejection particles and a depth where stress is applied(generated), that a high stress is applied to extremely shallow layersat the surface as the particle diameter of the ejection particles getssmaller. It is also apparent that although stress is input to deeperlayers as the particle diameter gets larger, this stress is lower.

In particular, it is apparent from FIG. 4 to FIG. 8 that the depth andintensity of the stress input to the surface of the metal articlechanges at a turning point of an ejection particle diameter of 20 μm.The intensity of the stress is greatly decreased when the ejectionparticle diameter exceeds 20 μm.

Namely, in the contour diagrams of FIG. 4 to FIG. 8, the center of theportions where a crescent shape can be seen represents the portion inputwith highest intensity stress. An extremely high stress was imparted toportions in the vicinity of the surface in the simulation of ejectionparticles of 20 μm or less. However, stress is spread out and disperseddeeply as the particle diameter increases, resulting in a weakerintensity of stress (see FIG. 9 and FIG. 10).

From the above results it is thought that indentations and protrusions(in particular protrusions), which are the cause of lamellar processingstructure formation as explained with reference to FIG. 1, are notliable to be formed on the surface of the metal article when ejectionparticles of 20 μm or less are employed. Moreover, employing suchejection particles is thought to result in an effect by whichcompositional strain exceeding the critical value required to generatethe nano-crystal structures is concentrated and generated in thevicinity of the surface of the metal articles.

Ferrous alloy ejection particles having a median diameter d50 of 20 μmwere ejected against regions of 6 mm×5 mm on test strips made from analloy tool-steel (SKD11), a pre-hardened steel (“NAK80”, manufactured byDaido Steel Co., Ltd), and an aluminum alloy (A7075). Changes in surfacehardness (dynamic hardness) were measured for each of the test strips.

In order to derive the ejection pressure suitably applied to each ofhard materials and soft materials, test strips were produced for each ofthe materials and treated at different ejection pressures. The dynamichardness was measured at 30 points in the regions of 6 mm×5 mm on thetest strips, and the found hardness taken as the surface hardness(dynamic hardness) of each test strip.

A graph of these measurement results is illustrated in FIG. 11.

Note that the dynamic hardness (DHT) is a hardness measured byindentation, and the conditions of measurement are as follows.

-   Test Instrument: Dynamic Ultra Micro Hardness Tester “DUH-W210”,    manufactured by Shimadzu Corporation-   Indentation Load: 3 gf (A7075), 5 gf (“NAK80”), 10 gf (SKD11)-   Time Held: 5 seconds-   Shape of Indenter: Triangular pyramid diamond indenter (115°)-   Computation Method DHT=α×P/(D²)

Note that in the above equation DHT is the dynamic hardness, a is anindenter shape coefficient (3.8584), P is the indentation load (mN), andD is the indentation depth.

Hitherto, it has been thought that raising the ejection pressure iseffective when attempting to impart intense stress to the surface of ametal article using shot peening.

However, from the measurement results of the dynamic hardness (DHT)illustrated in FIG. 11, even employing the ejection particles of thepresent invention having a median diameter of 20 μm or less, an increasein surface hardness (dynamic hardness) of a metal article according tothe rise in ejection pressure has been confirmed to be achieved in arange of ejection pressures from more than 0 MPa to 0.1 MPa. It was alsoconfirmed that a further rise in surface hardness was no longer seen forejection pressures exceeding 0.1 MPa, regardless of whether the teststrip was made from a high hardness material or a low hardness material,i.e. the hardness raising effect became saturated in the vicinity of anejection pressure of 0.1 MPa.

It is accordingly thought to be possible by the method of the presentinvention to impart the energy required to raise the hardness of thesurface of the metal article (and therefore to causenano-crystallization thereon) by treatment with an ejection pressure of0.05 MPa or greater. It was confirmed that it was possible to performsurface treatment of both hard materials and soft materials by using acomparatively low ejection pressure of not more than 0.5 MPa.

Moreover, due to being able to perform treatment employing such fineejection particles with a comparatively low ejection pressure, thedeformation of the metal article surface is suppressed to a minimum evenwhen treating a metal article made from a soft material, and it isthought that this enables the lamellar processing structures explainedwith reference to FIG. 1 to be suppressed from being generated.

In this manner, it is thought that the reason why a low ejectionpressure can be employed in the method of the present invention isbecause, although generally when particles in the air are caused tosettle out under gravity the particles settle out due to weight(external force) when the particle diameter is large, the particles arereadily carried on an airflow and have the property of not being liableto settle out when the particle diameter is small.

Namely, such ejection particles of small particle diameter have a smallmass and the influence of inertia is small. There is accordingly no needfor a large force to move such particles, and these ejection particlesare easily carried on an ejected airflow even when the pressure of thetransport gas is a low pressure. This enables the ejection particles tobe ejected from the ejection nozzle easily with a velocity close to thatof the compressed gas since the distance until the maximum velocity isachieved is short.

As a result, employing ejection particles that are easily carried on anairflow as stated above eliminates a large difference between theejection velocities of the ejection particles when ejected at anejection pressure of 0.1 MPa and when ejected at an ejection pressure of0.5 MPa. This is accordingly thought to lead to being able to obtain asimilar increase in hardness to that at an ejection pressure of 0.5 MPaeven when the ejection pressure is 0.1 MPa.

Moreover, a hardness that is not less than 60% of the hardness at 0.1MPa can still be imparted even when the pressure is 0.05 MPa.

However, even with ejection particles having a median diameter of 20 μmor less, those having a large mass are more readily influenced byinertia, are less liable to be carried on an airflow, and arrive at thesurface of the metal article prior to reaching the maximum velocity.

Thus the iron-based ejection particles having a median diameter of 20 μmemployed in the above tests have a falling time through air (inverse ofterminal velocity according to Stokes' Law or Stokes' equation) that is10.6 sec/m. In the tests employing such ejection particles, a good risein surface hardness (dynamic hardness) could be obtained for ejectionpressures within the range of from 0.05 MPa to 0.5 MPa.

It is accordingly thought that the required ejection velocity can beachieved as long as the falling time through air is longer than that ofthese ejection particles so that the ejection particles are readilycarried on an airflow, enabling nano-crystallization to be obtained atthe surface of the metal article.

From the results described above, the ejection particles employed inmethod of the present invention are determined to be ejection particleshaving a median diameter of not greater than 20 μm, and having a fallingtime through air of not less than 10 sec/m.

Note that, as seen from Table 2 and Table 3, the ejection velocity isnot less than 80 m/sec for the above described iron-based ejectionparticles having a particle diameter of 20 μm. Thus in the surfacetreatment method of the present invention, the ejection particles arepreferably ejected at an ejection velocity of not less than 80 m/sec.

Advantageous Effect Confirmation Tests

(1) Tests Objective

Performing shot peening under the treatment conditions obtained from theresults of the tests and simulations performed to derive the treatmentconditions as described above confirmed that a uniform nano-crystalstructure formation could be continuously formed along the surface ofboth metal articles made from hard materials and metal articles madefrom soft materials. It was also confirmed that a high residual stresscould be imparted to the surface of the metal article.

(2) Test Method

The surface treatment of the method of the present invention wasperformed on the test strips made from a pre-hardened steel (“NAK80”,manufactured by Daido Steel Co., Ltd), an alloy tool-steel (SKD11), andan aluminum alloy (A7075). The surface treatment conditions are listedin Table 4 below.

TABLE 4 Test Conditions NAK80 SKD311 A7075 Surface Blasting method SF SFSF Treatment Ejection particle Ferrous alloy Ferrous alloy Ferrous alloymaterial and median (Median diameter (Median diameter (Median diameterdiameter D50 (μm) D50: 20 μm) D50: 20 μm) D50: 20 μm) Ejection pressure  0.5   0.5   0.5 (MPa) Nozzle diameter (mm) φ7 φ7 φ7 Ejection time(sec) 30 30 30

(3) Observation Method

Each of the test strips that had been surface treated under theconditions described above was observed by the following method.

(3-1) SIM Observation

A scanning ion microscope (SIM) (“SMI3050SE”, manufactured by HitachiHigh-Tech Science Corporation) was employed to observe changes incrystal structure in the vicinity of the surface of each test strip.

(3-2) EBSD Observation

Electron back scatter diffraction analysis was employed (using anElectron Back Scatter Diffraction instrument manufactured by TSLSolutions Corporation) to observe crystal structure in the vicinity ofthe surface of each test strip, and to observe the crystal graindiameter and a crystal grain distribution therein.

(3-3) Residual Stress Measurements

A portable X-ray residual stress analyzer (“p-X360” manufactured byPulsetech Industrial Co., Ltd) was employed to measure the residualstress at the outermost surface layer of each of the test strips.

(4) Test Results

(4-1) Results of SIM Observations

FIG. 12 to FIG. 14 illustrate SIM images for each of the test strips.FIG. 12 is an SIM image for a pre-hardened steel (NAK80), FIG. 13 is anSIM image for an alloy tool-steel (SKD11). FIG. 14 is an SIM image foran aluminum alloy (A7075). In each of the respective drawings, thefigure appended with A was captured for test strips before treatment,and the figure appended with B was captured for test strips aftertreatment.

It could be confirmed for the test strips of all of the materials thatthe metal structure was clearly micronized in a zone down to about 3 μmfrom the surface layer after the surface treatment according to themethod of the present invention had been performed. The crystal grainsafter micronization were all confirmed to have a nano-crystal structure.

The nano-crystal structures were formed continuously along the surfaceof the test strips within the field of view of SIM mages (about 10 μm),and the formation of a continuous nano-crystal structure layer wasconfirmed.

Moreover, this nano-crystal structure, even for the test strip to betreated made from the aluminum alloy (A7075) which is a soft material,was confirmed to be formed as a uniform nano-crystal structure withoutcracks or the like occurring in the structure, and without beingaccompanied by the formation of the lamellar processing structuresexplained with reference to FIG. 1.

It was confirmed that in these test strips there was a region withsignificant fine-crystallization (nano-crystallization) in a zone downto 3 μm from the surface layer. There was also some micronizationobserved in a deeper zone of increased depth from the surface layer, andmicronization was particularly significant in the test strip made fromaluminum alloy.

The results of observations using SIM confirmed that the surfacetreatment method of the present invention was capable of forming auniform nano-crystal structure layer continuously along the surface,without being accompanied by the formation of the lamellar processingstructures, in a zone of a particular depth (about 3 μm) from thesurface for both test strips made from hard materials and test stripsmade from soft materials.

The test strips formed in this manner with a nano-crystal structurelayer in the vicinity of surface had, as explained with reference toFIG. 11, a surface hardness (dynamic hardness) is increased by about 100to 200 compared to untreated test strips (indicated at ejection pressure0 MPa in FIG. 11). This confirmed that the effectiveness as a method forstrengthening surfaces of metal articles formed from various materialsfrom soft materials through to hard materials.

(4-2) Results of EBSD Observations

The results obtained from EBSD analysis indicated a crystal graindiameter distribution in the vicinity of the surface of the pre-hardenedsteel (NAK80) test strip as illustrated in FIG. 15, and a crystal graindiameter distribution in the vicinity of the surface of the alloytool-steel (SKD11) test strip as illustrated in FIG. 16.

The results of observations using EBSD confirmed that the crystal graindiameter of the nano-crystal structure layer in the pre-hardened steel(NAK80) was in the range of from 100 nm to 500 nm. Moreover, the averagecrystal grain diameter in the crystal grain diameter distribution ofthis nano-crystal structure layer was found to be 240 nm (see FIG. 15).

In the alloy tool-steel (SKD11), the crystal grain diameter of thenano-crystal structure layer was confirmed to be in the range of from100 nm to 500 nm. Moreover, the average crystal grain diameter in thecrystal grain diameter distribution of this nano-crystal structure layerwas found to be 223 nm (see FIG. 16).

Note that in the aluminum alloy (A7075) test strip, the generatedcrystal grain diameter was much smaller than the resolution of EBSD.Thus, although crystallite analysis could not be performed by EBSD, dueto the highest resolution by EBSD being 30 nm, since the finest crystalgrains were observed in the test strips by SIM imaging, most of thecrystal grains can logically be presumed to mainly be smaller than the30 nm, which is the highest resolution of EBSD, in the nano-crystalstructure layer formed on the surface of the aluminum alloy (A7075). Thecrystal grain diameter of the nano-crystal structure layer formed on thesurface of the aluminum alloy (A7075) is accordingly thought to be 100nm or less.

(4-3) Residual Stress Measurement Results

The results of measurements of residual stress at the outermost surfacelayer of each of the test strips are summarized by graphs illustrated inFIG. 17 to FIG. 19.

In each of the test strips, residual stresses in the untreated statethat had positive values (tensional stress) flipped to negative values(compressional stress). The surface treatment method of the presentinvention was accordingly confirmed to be capable of imparting a highcompressive residual stress.

From among these test strips, the stress in the pre-hardened steel(NAK80) illustrated in FIG. 17 and the stress in the aluminum alloy(A7075) illustrated in FIG. 19 showed hardly any change by changes ofthe ejection pressure. This confirmed that sufficient compressiveresidual stress could be imparted by ejection at comparatively lowejection pressures of not more than 0.5 MPa, as long as an ejectionpressure of 0.1 MPa or above was achieved as stated above.

The residual stress of the aluminum alloy (A7075) is illustrated in thegraph of FIG. 19. This graph shows as a Comparative Example the resultsof residual stress measurements when ejection particles having a mediandiameter of 40 μm, this being larger than the range of the presentinvention, were ejected at an ejection pressure of 0.5 MPa.

Thus, in the example (Comparative Example) with ejection particleshaving a comparatively large particle diameter to the ejection particlesemployed in the present invention, although a compressive residualstress could be imparted, the residual stress that could be impartedthereby was ⅕ or less in compared with the residual stress imparted bythe method of the present invention. Therefore, when the method of thepresent invention was employed to perform surface treatment, a highersurface strengthening effect was obtained.

Note that in the test results for alloy tool-steel (SKD11) (see FIG.18), although an increase in residual stress with increasing ejectionpressure was observed, sufficient residual stress was still impartedeven in cases in which ejection was performed at the lowest ejectionpressure (0.1 MPa).

Moreover, in the example in which surface treatment of the presentinvention was performed at an ejection pressure of 0.1 MPa, although theresidual stress was slightly lower than that of the Comparative Example(ejection particles having a median diameter of 40 μm at an ejectionpressure of 0.5 MPa), a residual stress similar to or surpassing that ofthe Comparative Example was imparted at ejection pressures of 0.3 MPaand 0.5 MPa.

Application to Cutting-Edge of Machining Tool

(1) Test Method

Blanking punches made from SKD11 and having cutting-edge portionstreated with the surface treatment method of the present invention(Examples 1 and 2), a blanking punch made from untreated SKD11(untreated punch), and a blanking punch made from SKD11 surface treatedunder treatment conditions deviating from the treatment conditions ofthe present invention (Comparative Example 1) were employed for punchprocessing. The states of the cutting-edge portions were respectivelyobserved after processing.

(2) Surface Treatment Conditions

Surface treatment was performed under the conditions listed in Table 5below on a cutting-edge portion (the cutting-edge and a region up to 5mm from the cutting-edge) of each of the punches (length 3 cm, diameter0.5 cm) for punch-processing made from SKD11.

TABLE 5 Surface Treatment Conditions of a Punch for Punch-ProcessingComparative Example 1 Example 2 Example 1 Surface Ejection method SF SFSF treatment Ejection particle HSS (Median Aluminum (Median HSS (MedianMedian diameter diameter D₅₀: diameter D₅₀: diameter D₅₀: D₅₀ (μm) 15μm) 16 μm) 80 μm) Ejection pressure (MPa) 0.3 0.05 0.3 Nozzle diameter(mm) 7 7 7 Ejection duration (sec) 30 30 30

Note that “SF” for “Ejection method” in Table 5 indicates a suctionejection method employing a “SFK-2” manufactured by Fuji ManufacturingCo., Ltd. as the blasting apparatus in these test examples.

(3) Punch-Press Processing Conditions and Observation Method

Punches respectively surface treated with the methods of Example 1,Example 2, and Comparative Example 1, and an untreated punch, wererespectively employed to perform punch-press processing successively for9000 cycles on a workpiece made from SS steel. The surface state of eachof the punches after the punch-press processing had been performed wasthen observed by eye and with a microscope, and the state of wear noted.

(4) Observation Results

The surface state of each of the punches after the punch-pressprocessing is as listed in Table 6 below.

TABLE 6 Punch Surface State After Punch-press Processing TreatmentConditions Surface State Example 1 Hardly any observable damage. Nooccurrences of accumulation of material to be processed. Example 2Hardly any observable damage. No occurrences of accumulation.Comparative Multiple scratches having a striation shape. Example 1 alongthe length direction observed. Some accumulations of material to beprocessed were occurred. Untreated punch Unusable after 1800 cycles.

(5) Interpretation

In the present invention, performing the surface treatment of thepresent invention on punches made from SKD11 was seen to raise hardness,from a surface hardness of about 750 Hv when untreated to a hardness ofabout 950 Hv after surface treatment by the treatment of Example 1, thatis, an uplift in hardness of about 21%.

Moreover, the treatment of Example 2 was seen to raise hardness to about870 Hv, that is, an uplift in hardness of about 16%.

Such an uplift in hardness is thought to have been achieved due to theformation of the nano-crystal structure layer described above.

Moreover, the punches treated with the surface treatment methodaccording to the present invention (Examples 1 and 2) were capable ofpreventing material to be processed from accumulating to thecutting-edge as described above. This is thought to be a reason why goodpunching performance was exhibited over a prolonged period of time, anda reason why the lifespan of the punches was raised.

The mechanism obtaining the effect of preventing accumulation of cutmaterial is not entirely clear. However, fine dimples (see FIG. 2(B))having an equivalent diameter of from 1 μm to 18 μm and a depth of from0.02 μm to 1.0 μm or less than 1.0 μm were formed on the surface of themetal article treated by the surface treatment method of the presentinvention. The projected area of these dimples is at least 30% of thesurface area of the region to be treated. It is thought that the effectof preventing accumulation of cut material is obtained because thesedimples are served as oil reservoirs.

Note that the diameter (equivalent diameter) and depths of the dimpleswere measured using a profile analyzing laser microscope (“VK-X250”manufactured by Keyence Corporation). Measurements of the metal articlesurface were made directly in cases in which direct measurement waspossible. In cases in which direct measurement was not possible, methylacetate was dripped onto a cellulose acetate film to cause the celluloseacetate film to conform to the metal article surface, and aftersubsequently drying and peeling off the cellulose acetate film,measurement was performed based on the inverted dimples transferred tothe cellulose acetate film. Surface image data imaged by the profileanalyzing laser microscope (or, image data processed to invert capturedimages measured by employing the cellulose acetate film) was analyzedusing a “Multi-File Analysis Application (VK-H1XM by KeyenceCorporation) to perform the measurements.

The “Multi-File Analysis Application” is an application that uses datameasured by a laser microscope to measure surface roughness, lineroughness, height and width, etc. The application analyzes theequivalent circular diameter, depth, and the like, sets a referenceplane, and is capable of performing image processing such as heightinversion.

In measuring, first the “image processing” function is used to set thereference plane (however, in cases in which the surface shape is acurved plane, the reference plane is set after the curved plane has beencorrected to a flat plane by using plane shape correction). Then, themeasurement mode is set to indentation in the “volume/area measurement”function of the application, indentations are measured with respect tothe set “reference plane”, and the “average depth” in the indentationmeasurement results and the average value of the results for “equivalentcircular diameter” are set as the depth and equivalent diameter of thedimples.

Note that the reference plane described above was computed from heightdata using a least squares method.

Moreover, the “equivalent circular diameter” and the “equivalentdiameter” mentioned above are measured as the diameter of a circledetermined by converting the projected surface area measured for anindentation (dimple) into a circular projected surface area.

Note that the “reference plane” described above indicates a flat planeat the origin (reference) measurement for height data, and is employedmainly to measure depth, height, etc. in the vertical direction.

Application to Sliding Member

(1) Test Method

Three types of flat sheets of SUS304, size 40 mm×40 mm and thickness 2mm, were prepared: sheets treated by the present invention (Example 3):untreated sheets having a mirror finish (Comparative Example 2); andsheets treated by related art (Comparative Example 3). The slidabilityof the sheets was then evaluated by friction-wear tests.

TABLE 7 Surface Treatment Conditions Example 3 Comparative Example 3Ejection method SF SF Ejection particle Ferrous alloy HSS (Mediandiameter Median diameter (Median diameter D₅₀: 40 μm) D₅₀ (μm) D₅₀: 20μm) Ejection pressure (MPa) 0.1 0.3 Nozzle diameter (mm) 7 7 Ejectionduration (sec) 20 20

(2) Evaluation Method

Ball-on-disc tests were performed on the SUS304 sheets treated under theconditions described above until a friction coefficient of 2.0 wasachieved. The times until this occurred were measured and compared toevaluate slidability.

TABLE 8 Friction-Wear Test Conditions Test Instrument FPR-2000 Load (g)10 Rotation diameter (mm) 4 Rotation speed (rpm) 200 Lubrication NoneGauge head 3/16 inch SUS304 ball

A ball-on-disc friction-wear tester was employed. A ball of 3/16 inchdiameter made from SUS304 was employed therein.

(3) Evaluation Results

A graph of measured changes to friction with respect to elapsed time isillustrated in FIG. 20.

As is apparent from these measurement results, when treatment wasperformed with the conditions of Example 3, the durability was about 5times high in comparison with the untreated one (Comparative Example 2),or about 3 times high in comparison with the one treated with theconditions of Comparative Example 3.

(4) Interpretation

It could be presumed that the testing had been performed with acommensurately low friction due to obtaining the durability of about 5times high in comparison with the Comparative Example 2 and about 3times high in comparison with Comparative Example 3. Thus performing thetreatment of the present invention is thought to obtain about 3 timeshigh in the slidability.

1. A method for surface treatment of a metal article comprising:ejecting substantially spherical ejection particles having a mediandiameter d50 of from 1 μm to 20 μm and a falling time through air of notless than 10 sec/m against a metal article at an ejection pressure offrom 0.05 MPa to 0.5 MPa; forming a nano-crystal structure layercontinuously along a surface of the metal article in a zone to aprescribed depth from the surface of metal article by uniformmicronization to nano-crystals having an average crystal grain diameterof not greater than 300 nm; and imparting compressive residual stress tothe surface of the metal article.
 2. The method for surface treatment ofthe metal article according to claim 1, wherein the ejection velocity ofthe ejection particles is not less than 80 m/sec.
 3. The method forsurface treatment of the metal article according to claim 1, wherein thematerial of the metal article is either aluminum or an aluminum alloy,and the crystal grain diameter of the nano-crystal structure layer ismicronized to a crystal grain diameter of not greater than 100 nm. 4.The method for surface treatment of the metal article according to claim1, wherein: the metal article is a machining tool, and a region to betreated is a cutting-edge of the machining tool and the vicinity of thecutting-edge; and dimples having an equivalent diameter of from 1 μm to18 μm and a depth of from 0.02 μm to 1.0 μm or less than 1.0 μm areformed on the region to be treated by ejecting the ejection particles,such that a projected surface area of the dimples occupies not less than30% of a surface area of the region to be treated.
 5. The method forsurface treatment of the metal article according to claim 1, wherein:the metal article is a sliding member; at least a sliding portion of thesliding member is a region to be treated; and dimples having anequivalent diameter of from 1 μm to 18 μm and a depth of from 0.02 μm to1.0 μm or less than 1.0 μm are formed on the region to be treated byejecting the ejection particles, such that a projected surface area ofthe dimples occupies not less than 30% of a surface area of the regionto be treated.
 6. A metal article comprising: a base metal having ahardness not greater than HV714; and a nano-crystal structure layerformed continuously along a surface of the metal article in a zone to aprescribed depth from the surface of metal article by uniformmicronization to nano-crystals having an average crystal grain diameterof not greater than 300 nm; and a compressive residual stress beingimparted to the surface of the metal article.
 7. The metal articleaccording to claim 6, wherein the metal article is configured fromeither aluminum or an aluminum alloy, and a crystal grain diameter ofthe nano-crystal structure layer is not greater than 100 nm.
 8. Themetal article according to claim 6, wherein: the metal article is amachining tool; the nano-crystal structure layer is formed on a surfaceof a region to be treated including a cutting-edge and a vicinity of thecutting-edge; and dimples having an equivalent diameter of from 1 μm to18 μm and a depth of from 0.02 μm to 1.0 μm or less than 1.0 μm areformed such that a projected surface area of the dimples occupies notless than 30% of a surface area of the region to be treated.
 9. Themetal article according to claim 6, wherein: the metal article is asliding member; the nano-crystal structure layer is formed on a surfaceof a sliding portion of the sliding member that makes sliding contactwith another member; and dimples having an equivalent diameter of from 1μm to 18 μm and a depth of from 0.02 μm to 1.0 μm or less than 1.0 μmare formed such that a projected surface area of the dimples occupiesnot less than 30% of a surface area of the region to be treated.
 10. Themethod for surface treatment of the metal article according to claim 2,wherein the material of the metal article is either aluminum or analuminum alloy, and the crystal grain diameter of the nano-crystalstructure layer is micronized to a crystal grain diameter of not greaterthan 100 nm.
 11. The method for surface treatment of the metal articleaccording to claim 2, wherein: the metal article is a machining tool,and a region to be treated is a cutting-edge of the machining tool andthe vicinity of the cutting-edge; and dimples having an equivalentdiameter of from 1 μm to 18 μm and a depth of from 0.02 μm to 1.0 μm orless than 1.0 μm are formed on the region to be treated by ejecting theejection particles, such that a projected surface area of the dimplesoccupies not less than 30% of a surface area of the region to betreated.
 12. The method for surface treatment of the metal articleaccording to claim 2, wherein: the metal article is a sliding member; atleast a sliding portion of the sliding member is a region to be treated;and dimples having an equivalent diameter of from 1 μm to 18 μm and adepth of from 0.02 μm to 1.0 μm or less than 1.0 μm are formed on theregion to be treated by ejecting the ejection particles, such that aprojected surface area of the dimples occupies not less than 30% of asurface area of the region to be treated.